玛纳斯河流域长期连作棉田土壤重金属剖面分布特征分析

Profile distribution of heavy metals in soil from long-term continuous cropping cotton field in Manas River Basin

以新疆玛纳斯河流域长期连作棉田土壤为材料,对长期连作棉花土壤As、Cd、Cr、Ni、Cu和Pb的含量和剖面分布特征进行研究,旨在为该地区合理施肥和棉田土壤环境质量保护提供理论依据。选取研究区荒地和连作棉花5、10、20、30 a的土壤,分5层采集1 m土壤样品(每层20 cm),分析土壤重金属As、Cd、Cr、Ni、Cu和Pb的含量和垂直分布特征,并对土壤重金属的来源进行了解析。结果表明:研究区棉田和荒地土壤重金属As、Cd、Cr、Ni、Cu和Pb含量均未超过国家土壤环境质量二级标准,Cr、Ni、Cu、Cd和As的含量明显高于新疆土壤背景值,除Pb元素外,棉田其他5种重金属含量均显著高于荒地;变异系数棉田大于荒地,Cd的变异系数最大为0.46。棉田和荒地土壤剖面重金属含量都表现出表层略高于底层,随着连作年限的增加,土壤重金属含量显著增加且表现出更为明显的表层富集趋势,对比荒地和连作30 a的各层土壤,Cr、Cu、Ni、Pb、As和Cd含量增加幅度分别在36.68%~102.81%、43.31%~74.33%、21.85%~64.95%、2.12%~18.94%、23.03%~45.68%和167.50%~269.19%之间,总体来看,棉田土壤重金属在1 m土体中均表现出不同程度的累积。棉田土壤重金属含量相关性分析表明,土壤P素含量与棉田土壤中As、Cd、Cr、Cu和Ni呈极显著和显著相关,其相关系数分别为0.622、0.689、0.611、0.513和0.560,棉田因子载荷矩阵第一主成分As、Cd、Cr、Cu、Ni、Pb和P具有较大的载荷,分别为-0.875、-0.952、-0.868、-0.907、-0.964、-0.635和-0.864,累积贡献率达到了85.46%,说明化学磷肥的长期大量施入,是棉田土壤重金属含量累积的主要原因。

The content of heavy metals in soil is one of the best indicators for rational fertilization and environmental protection. The long- term continuous cropping of cotton is a common planting pattern in the Manas River Basin, Xinjiang Province, and it may increase the con- tent of heavy metals in the soil. To gain a better understanding of heavy metal accumulation caused by this common planting pattern, soil samples were collected from wasteland and from continuous cropping cotton fields that had been cultivated for varying periods（5, 10, 20, or 30 years ）. Soils samples were taken for each cultivation period （20 cm square by 1 m vertical depth）. We then determined the content, the vertical distribution characteristics, and the source of the heavy metals（As, Cd, Cr, Ni, Cu, and Pb） in the soil samples. The results showed that the contents of heavy metals in the cotton fields and wasteland did not exceed the secondary standard of national soil quality. All the av- erage contents of the heavy metals in the cotton fields were apparently higher than both the background values for the Manas River Basin and the wasteland values, with the exception of Pb content. The variable coefficient values for the cotton fields were higher than those for the wasteland, Cd was the maximum of the heavy metal elements, with a variable coefficient value of up to 0.46. The contents of the surface layer were only slightly higher than those of the under layer, and more heavy metals accumulated in the surface layer with increasing years of cultivation. Compared with the contents in every layer of wasteland, heavy metals accumulated in the cotton soil to varying degrees in the different soil layers, and the increased ranges were：36.68%-102.81%（Cr）; 43.31%-74.33%（Cu）; 21.85%-64.95%（Ni）; 2.12%-18.94% （Pb）, and 23.03%-45.68% （As）; Cd increased to the greatest extent, reaching 167.50%-269.19%. Correlation analysis revealed a highly significant correlation between the contents of P and As, Cd, Cr, Cu, and Ni, with correlation coefficients of 0.622, 0.689, 0.611, 0.513, and 0.560, respectively. In the cotton fields, the cumulative contribution rate of As, Cd, Cr, Cu, Ni, Pb, and P in the factor loading matrix was 85.46%, and the corresponding loadings in the first principal component were -0.875, -0.952, -0.868, -0.907, -0.964, -0.635, and -0.864, respectively. Our research indicates that extensive and long-term application of chemical phosphorus fertilizer is the main cause of heavy metal accumulation in the cotton fields of Xinjiang Province.